Recently, the demand for aluminum has been increasing in a wide range of industries, such as the home appliance industry and automotive industry, etc., since aluminum is lightweight, highly workable (high extensibility and forgeability), and has high thermal conductivity, etc. However, aluminum is soft, and is not suitable for practical use. Therefore, the surface of an article made of aluminum is generally anodized to form an anodic oxide film exhibiting good characteristics, such as hardness, corrosion resistance, abrasion resistance, adhesion, uniformity, and coloring, etc. The anodized aluminum film obtained is generally called “an alumite film”.
As one method for forming an anodic oxide film on aluminum, electrolysis in an acidic bath or an alkaline bath is generally known. Among the known methods, a sulfuric acid bath is most popular, and a film obtained using the sulfuric acid bath has the advantages of high corrosion resistance, abrasion resistance and also low manufacturing cost. Other acidic baths such as an oxalic acid bath, chromic acid bath, and a phosphoric acid bath, etc., are known.
Anodized aluminum treatment is carried out as follows; an object, used as an anode, is supplied with electricity in an electrolytic solution comprising sulfuric acid or oxalic acid, etc.; consequently a surface of the object is oxidized, generating Joule heat according to the following reaction formula;2Al3++3O−2→Al2O3(anodized aluminum film)and thus, an Al2O3 film (anodized aluminum film) is formed in the direction of depth. As shown in FIG. 1, the anodized aluminum film 2 formed on the object 1 expands in the volume at the treated surface 3, and grows upward and downward with respect to the untreated surface 4. Furthermore, as shown in FIG. 2, the film formed on the object 10 has a structure comprised of a porous layer 13 consisting of cells, each having a pore called a fine pore 11 and a cell diameter 12, and a barrier layer 14 under the porous layer.
Traditional anodization requires about one hour to form an anodized aluminum film of approximately 2.5 μm in thickness, for example, using a sulfuric acid bath, and has been strongly desired to be improved.
Japanese Unexamined Patent Publication No. 7-90688 discloses an attempt to shorten the anodization time to form an anodized aluminum film. In other words, Japanese Unexamined Patent Publication No. 7-90688 discloses a method of high-speed anodized aluminum treatment, wherein a surface of an object made of an aluminum alloy is subjected to a high-speed anodized aluminum treatment to form an oxide film thereon, characterized in that while an oxide film is formed on the surface of the member to be treated, the surface of the oxide film is flattened. However, in such an anodized aluminum treatment, it is necessary to provide in a cathode a portion to be treated in order to flatten the surface of the oxide film formed on the object, and accordingly there is a need of further improvement.
Japanese Unexamined Patent Publication No. 2005-304197 discloses a power supply device for anodization, which has a pair of pulse current generating circuits, and which variably controls positive and negative direct currents separately to supply high frequency pulses to a load circuit, so that an oxide film can be formed on an anode at a high speed and a high density. Japanese Unexamined Patent Publication No. 2007-154300 discloses a method for anodizing an aluminum alloy, characterized by short-circuiting an anode for anodization and a cathode for anodization when no pulse voltage is applied. Japanese Unexamined Patent Publication No. 2007-154301 discloses a method for anodizing an aluminum alloy characterized by performing anodization at a frequency corresponding to the lowest electrolytic voltage. Japanese Unexamined Patent Publication No. 2007-154302 discloses a power supply system for anodizing an aluminum alloy, comprising a condition input means, a control means which controls the pulse shape and pulse duty ratio according to a change of the electrolytic voltage or electrolytic current density in accordance with the progress of the anodization, and a power supply unit.
Japanese Unexamined Patent Publication No. 2004-35930 discloses a method of anodization of an aluminum alloy, wherein an aluminum or aluminum alloy object is immersed in a treatment bath, and is supplied with a high frequency current from 200 to 5000 Hz to anodize the aluminum or aluminum alloy. More specifically, this patent publication describes that, for example, the concentration of the sulfuric acid treatment bath is preferably from 3 to 30%, or an oxalic acid bath preferably from 1 to 5%, and the temperature of the treatment bath is preferably from −5 to 40° C. in case of a sulfuric acid bath or preferably from 10 to 60° C. in case of an oxalic acid bath.
Japanese Unexamined Patent Publication No. 2007-204831 discloses a method of a high-speed formation of an anodic oxide film of aluminum or an aluminum alloy having a thickness of 150 μm or more, wherein electrolysis is performed by applying an alternating current superimposed on a direct current, and time-dependent controlling the electrolytic current density. This patent publication specifically describes that the basic current density is in the range from 0.5 to 20 A/dm2, and the amplitude is in the range from 0.5 to 15 A/dm2, individually in the course of electrolysis, and the acidic bath is a sulfuric acid bath or an oxalic acid bath.
However, the methods of anodization described in Japanese Unexamined Patent Publication Nos. 2005-304197, 2007-154300, 2007-154301, 2007-154302, 2004-35930 and 2007-204831 have, for example, a drawback that a dedicated device e.g., a device variably controls positive and negative direct currents separately, or a device supplies a high frequency pulse current to a load circuit, must be provided.
Regarding the anodization of aluminum using an oxalic acid bath, WO97/35716 discloses a method for manufacturing a thermoplastic resin-coated aluminum alloy sheet, wherein an aluminum alloy sheet, as a substrate, is treated with an alkaline solution, followed by treatment with an acidic solution, so that a surface of the sheet has an increasing rate of 3 to 30%, of the specific surface area and then is subjected to anodization treatment, and further, a thermoplastic resin layer is laminated on at least one side of the substrate. In this method, the aluminum alloy sheet which has been subjected to the acid treatment is anodized using an acidic solution comprising, as a main component, one acid or two or more acids, selected from sulfuric acid, phosphoric acid, carboxylic acid and peroxycarboxylic acid, of 10 to 100 g/L under the conditions of a temperature from 30 to 80° C. and a current density from 2.5 to 50 A/dm2. More specifically, describes that the thickness of the oxide film formed by the anodization is 2 to 10 nm, and the carboxylic acid is oxalic acid or acetic acid.
Japanese Unexamined Patent Publication No. 2003-328187 discloses a method for surface-treating an aluminum material to form an anodic oxide film on a surface of the aluminum material made of aluminum or an aluminum alloy, wherein anodization is performed in an electrolytic bath consisting of an oxalic solution at a final voltage from 100 to 550 V, and then heat treatment is performed at an ambient temperature from 100 to 300° C. for 15 to 300 minutes. Furthermore, it is also described that one or two or more factors selected from an oxalic acid concentration from 5 to 80 g/L in the electrolytic bath, a bath temperature from 0 to 15° C., and an electrolytic current density from 2 to 10 A/dm2 are used in the anodization.
Japanese Unexamined Patent Publication No. 2005-29891 discloses a method for manufacturing a surface-treated aluminum material, comprising a step of anodizing an aluminum substrate made of aluminum or an aluminum alloy which is immersed in an electrolytic solution, and supplied with electricity, and a step of sealing the pores of the aluminum substrate using pressurized steam or high-temperature water of 95° C. or more, wherein the concentration of dissolved aluminum in the electrolytic solution is 0 to 5 g/L, and the electrolytic solution contains 10 to 50 g/L of any one acid or a mixture thereof selected from oxalic acid, malic acid, melonic acid, malonic acid, or tartaric acid, and the temperature of the electrolytic solution is 5 to 20° C.
However, in the methods of anodization using the oxalic acid solution as an electrolytic solution, as described in WO97/35716 and Japanese Unexamined Patent Publication Nos. 2003-328187 and 2005-29891, it is difficult to shorten the anodization time to form an anodized aluminum film of approximately 2.5 μm in thickness, and there is a need for further improvement.
Japanese Unexamined Patent Publication No. 5-24377 discloses a method for anodizing a support for a planographic printing plate, wherein an elongated aluminum or aluminum alloy strip is supplied with electricity, and anodized by electrodes provided in an electrolytic solution in which the strip is moved, and the electrolytic solution flows at a flow rate of at least 200 mm/second.
Japanese Unexamined Patent Publication No. 9-217200 discloses an anodizing device for aluminum or an aluminum alloy, having an anode circuit which is provided with a current distribution resistor, wherein the electrolytic solution is distributed at a uniform flow rate, and is uniformly injected from discharge slits of a rotary injector onto a surface of an object used as the anode.
Japanese Unexamined Patent Publication No. 11-236696 discloses a method of high-speed anodization of aluminum, wherein under control of the flow rate of the solution circulated and stirred in an electrolytic bath so that the flow rate relative to an object is regulated to be 30 cm/second or more, and 300 cm/second or less, a prescribed initial current density is controlled to be low to anodize the object.
Japanese Unexamined Patent Publication No. 2004-43873 discloses a method for surface treatment of an aluminum alloy, wherein while a circulation operation in which an electrolytic solution discharged from a discharge opening flows rotating around a member to be treated in an electrolytic tub, is then discharged therefrom, and is returned to the discharge opening is performed, the member to be treated is supplied with electricity and anodized.
Japanese Unexamined Patent Publication No. 2005-68458 discloses a method for anodizing an aluminum alloy wherein a hole slightly larger than the cross-section of an object is provide in an electrolytic tub; the object is inserted into the hole so that a treating portion of the object on which an anodic oxide film is to be formed is located in the electrolytic tub, whereby a gap is formed between the hole and the object at the boundary between the treating portion and the remaining portion of the object; air is blown through an opening provided on the outside of the electrolytic tub from all directions of the outer circumference of the member, onto the boundary of the member to remove the electrolytic solution discharged through the gap from the electrolytic tub.
Japanese Unexamined Patent Publication No. 2005-314751 discloses an anodizing device having a sealing member to close an opening of a hollow part of an object to be treated, and an electrode placed in the hollow part, wherein the electrode is made of a hollow body and has an outlet of an electrolytic solution which extends from a hollow chamber of the electrode toward the inner part in a direction oblique to the tangential direction.
Japanese Unexamined Patent Publication No. 2006-336050 discloses an anodizing device having a first electrode which energizes a metallic object to be treated with a ring-shaped recess on the circumferential face, a second electrode having an electrolytic solution passage, an inner circumference surrounding the outer circumference of the object, and an outlet opposed to the recess and formed in the inner circumference to communicate with the electrolytic solution passage, an electrolytic solution supply means for supplying the electrolytic solution passage to the electrolytic solution and for spraying the electrolytic solution through the outlet toward the recess, and an energizing means for applying a voltage to the first electrode and the second electrode.
However, in the methods described in Japanese Unexamined Patent Publication Nos. 5-24377, 9-217200, 11-236696, 2004-43873, 2005-68458, 2005-314751 and 2006-336050, it is necessary to provide a high-speed liquid flow or a nozzle for a uniform liquid flow, etc, and consequently the apparatus is too complicated for treatment using a simple tub, and therefore further improvement is necessary.
Generally, in order to speed-up the anodized aluminum treatment, it is necessary to increase the current density. Because the increase of the current density increases a surface temperature of an object, it is common to lower the surface temperature of the object by enhancing the stirring or aeration of liquid, etc. However, it is difficult to uniformly cool a surface of an article having a complicated shape by flow of or aeration of liquid, etc. In particular, an oxalic acid bath which generates a large amount of heat creates irregular surface temperatures. Consequently, the thickness of the anodized aluminum film becomes thick at a high-temperature part and thin at a cooled part, thus leading to irregular thicknesses. In an extreme case, the film opposite, of the liquid flow is increased and burned due to overheating.